1. Field of Invention
This invention pertains generally to the characterization and analysis of aromatic mixtures, and more particularly to human perceptual characterization and analysis of aromatic mixtures using in-instrument gas chromatography recomposition-olfactometry methods and apparatus.
2. Background
Aromas are typically mixtures of small volatile molecules that are able to stimulate a physiological response in low concentrations. Volatile chemicals that are present in inhaled air stimulate olfactory receptors and nerves in the nasal epithelium producing a signal that is transmitted to the brain. Processing of the signal by the brain includes a comparison with previously experienced smells, accounting for the high subjectivity seen in human aroma perceptions. Aromas also play a dominant role in the multisensory perception of flavor.
The mammalian olfactory system is a highly complex and efficient odor detection system derived from millions of years of evolutionary development. More than a thousand olfactory genes have been identified that produce receptor proteins involved in the interpretation of aromas and account for about two percent of the known genes in the genome.
Large numbers of olfactory receptors provide a system that is capable of detecting and discriminating between many different aroma compounds because the receptors have a broad sensitivity and may be activated by different aroma molecules with similar molecular structures. In addition, the majority of aroma molecules are capable of activating more than one type of olfactory receptor. Consequently, different combinations of receptors may be activated by a particular aroma molecule. The possible combinations of activated receptors are substantial so that the overall olfactory system is able to detect thousands of different molecules, even those that have not been encountered before.
The experienced perception of the group of aroma compounds that is sensed from the environment is considered to be the smell or aroma of a particular material. The aroma-derived components of flavors are perceptual constructs created by the brain in response to stimulation of the olfactory system as the brain detects and interprets mixtures of volatile chemicals and mixtures.
Quantifying the relationship between aroma perception and chemical composition in complex mixtures such as foods and beverages has been complicated by the observed complexity of aroma mixtures, the subjective olfactory memory and description standardizations. Generally, the chemical analysis of mixtures of odorous compounds can be complicated due to the phenomena of synergy, inhibition and masking that can take place between different compounds. Furthermore, when mixtures of aroma molecules combine, some of the aroma components in the resulting mixture may delete or accentuate the contributions of other aroma molecules creating unexpected perceptual results.
Approaches that are based on analytical chemistry for characterizing aromas or flavors typically rely on separation-based chromatographic methods that quantify the aroma strength of individual compounds in a mixture, reflected as either the concentration present in the mixture divided by a measured sensory threshold concentration (Odor Activity Value, OAV) or the number of N-fold dilutions required to suppress detectability of a compound when analyzed by gas chromatography with a human subject acting as an olfactory detector (GC-Olfactometry or GC-O; CHARM; or Aroma Extract Dilution Analysis). Reconstitution and omission experiments evaluate the role of specific compounds in the perceived aroma of a mixture, whereby a blend of compounds hypothesized to be detectable in a food, beverage, or other sample by OAV is mixed from chemical standards, and compared to similar mixtures prepared by omitting one of these compounds at a time. If a difference is detectable in the “whole” mix versus a “whole-minus-one-compound” mix, that particular compound is considered important to the aroma of the sample.
Knowledge acquired from other disciplines studying aromas, such as sensory psychophysics, cognitive psychology, and molecular neurobiology, suggest that there are limitations in these methodologies. Chromatographic techniques only assess the aroma quality of individual compounds, rather than mixtures of compounds. However, the aroma of a mixture is frequently perceptually distinct from that of its individual components and may have qualities not found in any of these individual components. The mixing-dependent nature of aroma quality is evidenced by the relative lack of aroma impact compounds, or those compounds that are singularly responsible for the overall aroma impression of a food or beverage. On the other hand, omission experiments rely on an assumption that all sensorially important compounds have been correctly identified and quantified and that any compound occurring at a concentration below its putative sensory threshold is not important to the overall aroma. However, this assumption is not always accurate. Despite having identical concentration profiles of supra-threshold odorants, the aroma of a reconstitution sometimes still smells different from the original mixture, a phenomenon referred to as “reconstitution discrepancy”.
Accordingly, there is a need for an apparatus and method of identifying and quantifying subsets of aroma compounds and the contribution of the compounds on the perceived aroma. There is also a need for an apparatus and method that is accurate and can reliably show the relation between the aroma mixture composition and olfactory perception regardless of concentration so that models can be created. The present invention satisfies these needs as well as others and is generally an improvement over the art.